Multiple-input and multiple output (MIMO) systems may apply multiple antennas at both the transmitter and the receiver in order to increase spectral efficiency. MIMO technology has been embraced by several recent wireless communications standards, including Institute of Electrical and Electronics Engineers (IEEE) 802.11ac (WiFi) and 3rd Generation Partnership Project (3GPP) Long Term Evolution (4G), where relatively small arrays of up to four antennas are used. MIMO is expected to continue to play a major role in emerging 5G systems, where it may be employed on a much more massive scale.